We plan to continue research into the biology of Biomphalaria glabrata, its relationship to Schistosoma mansoni, and its biological control, as follows: 1. To study the ultrastructure of the miracidium and early mother sporocyst with reference to: (1) structure and function of the miracidium's four types of peripheral sensory organelles; (2) fine structure of the neural ring, and histochemical assessment of its membrane-bound vesicles for certain biogenic amines; and (3) description and evaluation of the activities of snail amoebocytes that respond to the newly implanted sporocyst. 2. To determine the effects of external stresses, i.e., temperature, salinity, blood-loss, and desiccation, on the survival of the miracidium-sporocyst shortly after it becomes implanted in the snail. 3. To study the effects on B. glabrata of ionizing radiation with respect to: (a) changes in susceptibility to S. mansoni; (b) physico-chemical changes in hemolymph; (c) histopathology; and (d) variations in radiation sensitivity among several strains of the snail. 4. To evaluate the histopathology of tumor-like growths that appear as "genetic markers" in selectively bred stocks of B. glabrata. 5. To analyze the hemolymph of B. glabrata to assess alterations in non-specific esterases and dehydrogenases that accompany infection of the snail with S. mansoni. 6. To determine whether competition imposed by species-populations of schistosome-insusceptible pulmonate snails limits the growth and reproduction of B. glabrata. 7. To continue to search out and study parasites and predators which may be of potential value in the biological control of medically important snails.